diff --git a/src/attrs.c b/src/attrs.c
index 85d0751..84f9060 100644
--- a/src/attrs.c
+++ b/src/attrs.c
@@ -1367,11 +1367,8 @@ Bool TY_(IsValidHTMLID)(ctmbstr id)
     if (!s)
         return no;
 
-    if (!TY_(IsLetter)(*s++))
-        return no;
-
     while (*s)
-        if (!TY_(IsNamechar)(*s++))
+        if (TY_(IsHTMLSpace)(*s++))
             return no;
 
     return yes;
diff --git a/src/lexer.c b/src/lexer.c
index 522fcdc..04a139c 100644
--- a/src/lexer.c
+++ b/src/lexer.c
@@ -230,6 +230,11 @@ Bool TY_(IsLetter)(uint c)
     return (map & letter)!=0;
 }
 
+Bool TY_(IsHTMLSpace)(uint c)
+{
+    return c == 0x020 || c == 0x009 || c == 0x00a || c == 0x00c || c == 0x00d;
+}
+
 Bool TY_(IsNamechar)(uint c)
 {
     uint map = MAP(c);
diff --git a/src/lexer.h b/src/lexer.h
index 43b9b4a..d4d2958 100644
--- a/src/lexer.h
+++ b/src/lexer.h
@@ -416,6 +416,7 @@ void TY_(ConstrainVersion)( TidyDocImpl* doc, uint vers );
 Bool TY_(IsWhite)(uint c);
 Bool TY_(IsDigit)(uint c);
 Bool TY_(IsLetter)(uint c);
+Bool TY_(IsHTMLSpace)(uint c);
 Bool TY_(IsNewline)(uint c);
 Bool TY_(IsNamechar)(uint c);
 Bool TY_(IsXMLLetter)(uint c);